1. Field
The disclosed subject matter relates to a TV receiver called an LCD TV that can include an LED-based display, or a personal computer that can receive and record TV programs and can include a large LED-based display called a monitor, and more particularly to a backlight device for illuminating LCD displays from behind.
2. Description of the Related Art
An example of a conventional backlight device 90 for an LCD panel 80 is shown in FIG. 6. The backlight device 90 is operative to light two cold-cathode tubes 81, 82 provided on the LCD panel 80.
The cold-cathode tubes 81, 82 are connected to lighting circuits 91, 92 each composed of an inverter that applies a high voltage of, for example, a high frequency (several 10 kHz) for lighting. The lighting circuits 91, 92 are connected to a power circuit 83 for supplying power thereto.
The lighting circuits 91, 92 are also connected to a dimming controller 93, which provides the lighting circuits 91, 92 with dimming signals C1, C2 of several 100 Hz and with an appropriate duty ratio. The lighting circuits 91, 92 turn on the tubes when the dimming signal is at “H” level and turn off the tubes when the dimming signal is at “L” level. Thus, the duty ratio of the dimming signals C1, C2 can be varied to adjust the brightness of the LCD panel 80.
The LCD panel 80 is connected to a driver 84 that receives an image signal for displaying an image on the LCD panel 80 and drives the LCD panel 80. In addition, when no image signal is supplied, the driver halts the lighting circuits 91, 92 via the dimming controller 93 for saving power.
FIG. 7 shows the following: dimming signals C1, C2 that are output when the LCD panel 80 is driven; an output i1 from the cold-cathode tube 81 that lights in response to the dimming signal C1; an output i2 from the cold-cathode tube 82 that lights in response to the dimming signal C2; and a synthesized output (i1+i2) from both the cold-cathode tubes 81, 82. Driving the lighting circuits 91, 92 in this way makes it possible to keep the maximum of current flowing in the power circuit 83 unchanged while allowing the brightness of the screen to be changed. (For example, see Japanese Patent Document 1: JP-A 2002-50498).
In the above-described related art dimming system, a high frequency of 55-100 kHz for lighting is first applied to the cold-cathode tubes 81, 82. In addition, the dimming controller 93 performs PWM modulation for adjusting the brightness of the screen. When this method is used for dimming control, as schematically shown in FIG. 8, the energy corresponding to the sine wave of the high-frequency drive voltage for lighting can be spread to lower the peak value on the curve P to that on the curve Q as shown. This is an effective measure against electromagnetic radiation noises.
This method, however, disperses the infrared frequency components radiated from the cold-cathode fluorescent tube (CCFL) or the hot-cathode fluorescent tube (HCFL). In this case, the infrared frequency components are spread into the frequency range used by infrared remote controls that are used in video recording instruments such as TV receivers, video recorders and DVD drives to send data at frequencies near 38 kHz (see FIG. 3B). As a result, an adverse effect may be exerted on operation of the infrared remote control and, in an extreme case, a malfunction may occur in the infrared remote control.